1. Field of the Invention
The invention relates to a hydraulic power steering system that continuously provides a source for hydraulic assist to a steering gear while allowing the pump to be idled when there is no steering demand to reduce energy consumption.
2. Description of Related Art
Conventional hydraulic power assist steering systems employ an open center valve at the steering gear and a pump, which supplies constant flow through an open loop hydraulic circuit. Fluid is pumped through the system at all times while the engine is running regardless of steering load. By closing ports within the steering valve creating pressure internal to the gear and in the hydraulic circuit upstream to the pump, boost or steering assist is created. Running a hydraulic pump continuously in this type of system ensures good response to a steering input but is inefficient when there is no steering load. Restrictions in the hydraulic system can create significant backpressure against the pump and generate a significant amount of heat over time that must be managed with oil coolers. As a result, prior studies have shown that fuel consumption in the unloaded mode can dominate the fuel consumed under load.
U.S. Pat. No. 5,921,342 (the '342 patent) provides a Power Assisted Steering Apparatus for Automotive Vehicle, but does not have a proportional pressure relief for the pump under load but instead relies on throttling down flow through the system using a variable displacement pump when steering demand is low. Since the pressure drop across restrictions in the hydraulic system is proportional to flow rate, the backpressure that the pump needs to work against is lower when flow is reduced through the system in low steering demand situations. Therefore, this system reduces pumping losses and saves energy over hydraulic power steering systems that employ fixed displacement hydraulic pumps.
The patent application publication to Rogers et al. (2003/0127275 A1) provides for a High Efficiency Automotive Hydraulic Power Steering System and does have an automotive hydraulic power steering system that prevents wasted energy when no power assist is required but does not maintain proportional pressure control between the input side of the gear and the assist pressures internal to the gear. Proportional control is a key enabler to minimizing leakage through the steering valve and being able to control steering assist as a function of steering valve opening hydraulically without the need for electronic controls. The system described does not provide a load sensing signal downstream of the steering valve. Without a load sensing signal, the low pressure setting for the hysteresis pressure switch must be raised to ensure adequate response during worst case steering conditions which reduces the effective range of the accumulator.